Optical modulator/detector based on reconfigurable diffraction grating

ABSTRACT

A system is provided for positioning separate portions of a sample in elongate, parallel channels of a sample chamber and for irradiating a sample in the chamber to create a diffraction pattern where the sample and chamber differ in refractive index. The system also can measure absorption of electromagnetic radiation by a sample in the chamber, and can measure the absorption simultaneously with measurement of diffraction by the sample.

RELATED APPLICATIONS

[0001] This application is a divisional of U.S. patent application Ser.No. 09/425,787, filed Oct. 22, 1999, which is a CIP of U.S. Ser. No.09/274,372, filed Mar. 23, 1999, which claims priority to U.S. Ser. No.60/078,984, filed Mar. 23, 1998.

RIGHTS IN THE INVENTION

[0002] The invention described herein was supported at least in part bythe Defense Advanced Research Project Agency (DARPA) and by the NationalScience Foundation (NSF) under award ECS-9729405. The government hascertain rights in the invention.

FIELD OF THE INVENTION

[0003] The present invention relates generally to optical devices and,more particularly, to a microfluidic device defined by a plurality ofparallel, elongate channels for containing a sample and opticalapparatus for irradiating a sample in the channels and measuringabsorption of the sample and diffraction of electromagnetic radiation bythe system for sensor and modulation applications.

BACKGROUND OF THE INVENTION

[0004] Devices for spatial light modulation based on liquid crystals,colloidal crystal hydrogels, materials showing photochromic andphotoelastic effects, and micromachined structures are known. Some ofthese devices operate by exploiting differences in phase or absorptionof electromagnetic radiation.

[0005] Differences in phase of electromagnetic radiation can be producedby varying a path that the radiation follows, for example by passage ofthe radiation through media of differing refractive index. Phasedifferences of light have been exploited for a variety of uses includingsensors, apparatus for photolithography, optical displays, opticalcommunication, fiber optic carriers, and the like.

[0006] Absorption of material by electromagnetic radiation is determinedroutinely. Absorption of light by fluid samples typically is determinedby measuring absorption of light through an empty sample chamber, thenmeasuring absorption of the same light through the same chamber filledwith a sample, and comparing the two. A source of electromagneticradiation is positioned on a first side of the chamber and a detector ispositioned on the opposite side to detect radiation passing through thechamber, both filled and unfilled. These procedures are carried out withso-called absorption spectrometers which can determine absorption of asample over a range of electromagnetic radiation frequencies.

[0007] Solgaard, et al., in “Deformable Grating Optical Modulator”,Optics Letters 17, 9 (1992) describe a deformable light gratingmodulator based on electrically controlling the amplitude of amicromachined phase grating. The described structure includes siliconnitride beams suspended above a substrate. With no voltage appliedbetween the substrate and the individual beams, the total path lengthdifference for light reflected from the beams and from the substrateequals the wavelength of the incoming light. Reflections from the beamsand from the substrate add in phase, and the grating reflects the lightas a flat mirror. When a voltage is applied between the beams and thesubstrate, electrostatic force pulls the beams down. The total pathlength difference for the light reflected from the beams and from thesubstrate is now ½ of the wavelength, and the reflections interferedestructively, causing light to be diffracted. Solgaard, et al., statethat ease of integration is expected to make deformable mirrormodulators strong competitors with liquid crystal and electro-opticspacial light modulators in display technologies and for optical signalprocessing.

[0008] Delamarche, et al., “Microfluidic Networks for ChemicalPatterning of Substrates: Design and Application to Bioassays,” J. Am.Chem. Soc., 1998, 120, 500-508, describe a microfluidic network usefulfor the small-scale patterned delivery of reactants to surfaces usingthe elastomer poly(dimethylsiloxane) as at least a portion of thechannels. Woolley, et al., in “High-Speed DNA Genotyping UsingMicrofabricated Capillary Array Electrophoresis Chips,” AnalyticalChemistry, 69:11, Jun. 1, 1997, describe flowing a fluid throughmultiple channels and measuring optical characteristics of the fluid.

[0009] As mentioned above, measurement of properties of fluid samples insample chambers is carried out routinely. There are many uses forsystems that cause fluid to flow through channels, including very smallchannels. In an article entitled “Downsizing Chemistry,” Chemical &Engineering News, Feb. 22, 1999, 27-36, a variety of systems aredescribed for analyzing very small amounts of samples and reagents onchemical “chips” that include very small fluid channels and smallreaction/analysis chambers. Small-scale systems are currently beingdeveloped for genetic analysis, clinical diagnostics, drug screening,and environmental monitoring. Such small-scale systems are commonlyreferred to as Microscale Total Analysis Systems (see, for example,Kricka, L. J., Wilding, P. Micromechanics and Nanotechnology, inHandbook of Clinical Automation, Robotics and Optimization, Kost, G. J.,Welsh, J., Eds., John Wiley and Sons, New York, 1996, p. 45; van denBerg, A., Bergveld, P., Eds., Micro Total Analysis Systems, KluwerAcademic Publishers, London, 1995; Manz, A., Becker, H., Eds.Microsystem Technology in Chemistry and Life Sciences, Springer-Verlag,Berlin, Germany, 1998; Manz, A., Harrison, D. J., Verpoorte, E.,Wildmer, H. M. Adv. Chromatogr., 1993, 33, 1; Kricka, L. J., Wilding, P.Pure Appl. Chem., 1996, 68, 1831). Microscale Total Analysis Systemsmust be able to handle liquid or gas samples at very small scale, andmust be compatible with chip-based substrates. Microfluidics, thebehavior of fluid flow in very small-scale systems, therefore is centralto the development of these systems.

[0010] Miniaturized capillary electrophoresis systems are known.Capillary electrophoresis is a separation technique that,conventionally, is typically carried out in fused silica capillaries.Capillary electrophoresis within polymer channels also is known. For adiscussion of miniaturized capillary electrophoresis techniques, seeDuffy et al., “Rapid Prototyping of Microfluidic Systems inPoly(dimethylsiloxane), Analytical Chemistry, 70, 23, 4974-4984 (Dec. 1,1998), and references therein.

[0011] Electroosmotic flow is a known technique for urging the flow offluid within a channel by applying electric fields along the channels.Electroosmotic flow typically is carried out within glass channels inwhich it is relatively easy to create a negatively-charged interiorchannel surface to support electroosmotic flow.

[0012] Microfluidic systems based upon poly(dimethylsiloxane) are known.See Duffy, et al., referenced above, and references therein.

[0013] While the above and other reports describe in many cases usefuloptical modulators and sensors, and useful microfluidic systems, a needexists for simplified, inexpensive devices that can serve as effectivemodulators and/or can provide significant information when used as asensor and microfluidic systems that can be used in these and othertechniques. It is an object of the invention to provide such devices.

SUMMARY OF THE INVENTION

[0014] The present invention provides systems and methods for opticalmodulation and sensing. Some of these aspects and methods are describedin the accompanying claims.

[0015] A device of the invention can be a diffraction grating defined byessentially parallel lines. But the device is not limited to adiffraction grating with evenly spaced, parallel lines. By changing thewidth and spacing of the microchannels more complex light patterns canbe made, e.g. a variety of patterns defined by microchannels that changethe spatial distribution of light passing through the device throughdiffraction effects, through constructive and destructive interferenceof light passing through different regions of the device, such as aFresnel lens that focuses light using a planar array of channels.

[0016] Other advantages, novel features, and objects of the inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings,which are schematic and which are not intended to be drawn to scale. Inthe figures, each identical or nearly identical component that isillustrated in various figures is represented by a single numeral. Forpurposes of clarity, not every component is labeled in every figure, noris every component of each embodiment of the invention shown whereillustration is not necessary to allow those of ordinary skill in theart to understand the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a photocopy of a slightly magnified microfluidic deviceof the invention, including a photocopy of a more highly-magnifiedsection of channels of the device (inset);

[0018]FIG. 2 is a schematic cross-section of a microfluidic device ofthe invention that can function as a diffraction grating or absorptiondevice, and is a cross-section through lines 2-2 of FIG. 2A;

[0019]FIG. 2A illustrates schematically a top view of a microfluidicdevice of the invention in cross-section;

[0020]FIG. 3 illustrates schematically details of a grating of theinvention for purposes of describing diffraction and absorption;

[0021]FIG. 4 illustrates modulation of phase difference as a function ofindex of refraction of a fluid in a device of FIGS. 1-3 for differentchannel dimensions;

[0022]FIG. 5 shows variation of transmission coefficient of the deviceof FIGS. 1-3 as a function of concentration of an arbitrary absorbingspecies;

[0023]FIGS. 6 and 7 show variation in the intensities of 0th and 1storder diffracted beams as a function of phase and absorption of themicrochannels of the device of FIGS. 1-3 filled with fluid;

[0024]FIG. 8 shows variation in intensities of the 0th and 1st orderbeams of the diffraction pattern from a microfluidic grating of FIGS.1-3 as a function of concentration of NaCl in solutions filling themicrochannels;

[0025]FIG. 9 shows variation in the intensities of the 0th and 1st orderof the diffraction pattern from a microfluidic grating of FIGS. 1-3 as afunction of the concentration of Azure A in aqueous solutions;

[0026]FIG. 10 shows variation of transmission of the device of FIGS. 1-3(open circles) and phase (closed circles) as a function of concentrationof Azure A;

[0027]FIG. 11 is a cross-sectional schematic illustration of a device ofthe invention designed to measure both absorption and diffraction;

[0028]FIG. 12 is a schematic illustration of the effect of placingfluids of different refractive indices in different channels of thedevice of FIG. 10; and

[0029]FIG. 13 is a top, cross-sectional, schematic illustration of thedevice of FIGS. 1-3 and 10 in which different fluids have been placed indifferent channels and the fluids differ as a function of axial positionin each channel.

DETAILED DESCRIPTION OF THE INVENTION

[0030] The present invention provides, in one aspect, a sensor that cansimultaneously determine refractive index and absorption of a sample.

[0031] Referring to FIG. 2, a sensor system 10 is illustratedschematically including a first component 12 and a second component 14.First component 12 has a first surface 14 defining a plurality ofprotrusions 16 and indentations 18. Outward facing surfaces ofprotrusions 16 contact a surface 20 of second component 14. This definesa plurality of channels 22 each defined by indentations 18 and portionsof surface 20 in register with these indentations, which together definea sample chamber. Component 12 is shown in FIG. 2 in end view, orcross-section. The channels 22 are elongate channels that extend intoand out of the page.

[0032] B Components 12 and 14 are part of system 10 which defines asample chamber including the plurality of channels 22. Components 12 and20 are at least partially transparent to incident electromagneticradiation used in the techniques described below, and preferably isessentially completely transparent to such radiation.

[0033] If a sample contained within channels 22 has a refractive indexessentially identical to that of component 12, then when an incidentbeam of the electromagnetic radiation 24 impinges upon a sample withinchannels 22, the beam passes through un-diffracted. If a sample inchannels 22 has a refractive index different from the material ofcomponent 12, then incident beam 24 is diffracted and emerges asdiffracted beams 26. A detector can be positioned to detect whetherdiffraction occurs, and the extent to which diffraction occurs. Wherediffraction does not occur, all of the incident beam 24 emerges as beam27. That is, the beam is not diffracted. But where diffraction occurs,some of incident beam 24 is diffracted as diffracted beams 26 (1, 2, 3,−1, −2, and −3). Thus, a detector positioned to detect the existence andintensity of any of these diffracted beams 1, 2, 3, −1, −2, and −3 candetermine the existence and degree of diffraction. The opticalproperties (index of refraction; absorption) of the sample filling thechannels dictate the appearance of the diffraction pattern produced bythe grating. The period of the grating determines the angular positionof the diffracted beams, while the thickness of the channels and theoptical properties of the fluid determine their relative intensities.

[0034] The Fraunhofer (far-field) diffraction pattern can be calculatedby Fourier transformation of the transmission function of the grating.The transmission function, τ(x), of the microfluidic diffraction gratingis given by Eq. 1, where δφ is the difference in the phase of light thatpasses through the microchannels and the light that passes through PDMSonly, T is the transmittance of the fluid in the microchannels, w is thewidth of the microchannels, and p is the period of the grating:$\begin{matrix}{{\tau (x)} = \left\{ \begin{matrix}{\quad {\sqrt{T}^{\delta\phi}}} & {{kp} < x < {{kp} + w}} & {k\quad i\quad n\quad t\quad e\quad g\quad e\quad r} \\{\quad 1} & {o\quad t\quad h\quad e\quad r\quad w\quad i\quad s\quad e} & \quad\end{matrix} \right.} & (1)\end{matrix}$

[0035] Eq. 1 indicates that the transmission function and hence thediffraction pattern can be changed by either modulating the phase orchanging the amplitude of the light that passes through the microfluidicchannels. The modulation of the phase is determined by the profile ofthe index of refraction of the grating. The phase difference is given byEq. 2, where λ is the wavelength of the light, d is the depth of thechannels, and □n=n_(p)−n_(f), i.e., the difference between the index ofrefraction of PDMS (n_(p)˜1.41) and that of the fluid filling thechannels (n_(f)): $\begin{matrix}{{\delta\phi} = {\frac{2\pi}{\lambda}d\quad \Delta \quad n}} & (2)\end{matrix}$

[0036] By filling the channels with fluids that have different indicesof refraction, the phase of a given microfluidic device can therefore bemodulated. For examples: if empty channels are filled with water, thephase shift □Φ between the two states (filled vs. empty) isδΦ_(filled)−δΦ_(empty): this difference is equal to 21π or about 10modulation cycles at 633 nm for 20-μm thick channels; if the index ofrefraction of the fluid filling the channels matches that of PDMS, thenthe phase will not be modulated and the device will transmit lightwithout diffracting it. Eq. 2 shows that the modulation of phase iseffected by the thickness of the microfluidic channels: to illustratethis effect, |δΦ/2π| is plotted as a function of the index of refractionof the fluid filling channels for three different channel thicknesses inFIG. 4. Each unit on the y-axis of this plot represents one cycle in themodulation of the phase of the microfluidic diffraction grating as theindex of refraction of the fluid changes. FIG. 4 shows that diffractiongratings with thicker channels will be more sensitive than thinnerchannels to small changes in the index of refraction of a fluid.Specifically, FIG. 4 shows modulation of the phase difference, δΦ, as afunction of the index of refraction of the fluid for diffractiongratings with channels that are 10, 20, and 50 μm thick at 633 nm: achange in δΦ of 2 π represents 1 cycle of modulation. When the index ofrefraction of the fluid matches that of PDMS (n_(p)˜1.41), the device iscompletely transparent and no diffraction takes place. The sensitivityof the device to changes in index of refraction of the fluid increaseswith the thickness of the microchannels. The inset shows how the phasedifference varies with small changes in index of refraction.

[0037] If the channels of a microfluidic diffraction grating are filledwith a fluid that absorbs light, then the amplitude as well as the phaseof the light passing through the microchannels is effected. Eq. 1indicates that, in this case, the intensities of the diffracted beamswill depend on both the difference in phase and the amount of light thatis absorbed by the fluid. For example, a difference in phase of πresults in total extinction of the 0th order diffracted spot only whenthe beams that interfere destructively have the same amplitude; if theamplitude of the light passing through the fluid is changed due toabsorption, then there will not be complete destructive interference andthe intensity of the 0th order diffracted spot will be non-zero eventhough the difference is phase is π.

[0038] The influence of the geometry of the microchannels filled with anabsorbing liquid on the transmittance, T, is illustrated by theBeer-Lambert law (Eq. 3) that describes the effect on T of an absorberat low concentrations. In Eq. 3, I₀ and I are the intensities of theincident and transmitted beams, respectively, ε is the molar absorptioncoefficient, [C] is the concentration of absorbing species in thesolution, and d is the thickness of the microchannels: $\begin{matrix}{\frac{I}{I_{0}} = {T = 10^{{- {ɛ{\lbrack C\rbrack}}}\quad d}}} & (3)\end{matrix}$

[0039] Eq. 3 shows that the transmittance decreases exponentially as ε,[C], or d increases. FIG. 5 illustrates the variation of thetransmission of microfluidic diffraction gratings as a function ofconcentration of an absorbing fluid and the thickness of themicrochannels. Small changes in concentration (at low values ofconcentration) have a strong effect on the transmission of the device,and hence on the diffraction pattern, for thick microchannels. FIG. 5shows variation of the transmission coefficient of the microfluidicdiffraction grating, from values of T (the transmittance of thechannels) calculated using the Beer-Lambert law (Eq. 3), as a functionof the concentration of an arbitrary absorbing species, assumed to haveε˜10⁵ M⁻¹cm⁻¹, in the microchannels. As the thickness of themicrochannels increases, the transmission decreases: Eq. 1 indicatesthat this decrease will change the relative intensity distribution ofthe diffraction pattern.

[0040] In general, the microfluidic diffraction grating will not operateas a pure phase or amplitude grating: the variation in the intensitiesof the diffracted beams will be determined by changes in both δΦ and T.We determined numerical solutions of the Fourier transformation of Eq. 1to model the behavior of the gratings as a function of the opticalproperties of the fluid. The variations in the intensities of the 0thand 1st orders diffracted beams as a function of both phase andabsorption are plotted in FIGS. 6 and 7. The intensity of the 0th (1st)order beam is maximum (minimum) when the depth of modulation of thephase is 0 or 2π, and minimum (maximum) when the depth of modulation ofthe phase is 7π. If the index of refraction of a non-absorbing fluidfilling the channels varies over a sufficiently wide range so that theresulting shift in phase is ˜π, the optical response of a microfluidicdiffraction grating is switched between two extremes: i.e., from a statein which the 0th order beam is fully illuminated to one in which it isfully extinct. If the absorption of the fluid in the microchannelschanges, the relative intensities of the various diffracted orderschange: for example, the maximum contrast between 9th and 1st orderdiffracted beams decreases as the absorption increases. Specifically,FIGS. 6 and 7 show variation in the intensities of the 0th (6) and 1st(7) order diffracted beams as a function of the phase and absorption(1−{square root}{square root over (T)}) of the microchannels filled withfluid. These plots were generated by Fourier transformation of Eq. 1.

[0041] The device illustrated schematically in FIG. 2 is useful in avariety of situations. For example, system 10 can be used to determinethe existence or concentration of an analyte carried in a fluid wherethe analyte changes the refractive index of the fluid. If a fluidcarrier is passed through channels 22, and the fluid contains a varyingdegree of an analyte that changes the refractive index of the carrierfluid, then this difference in refractive index will result in a changein intensity of diffracted beams 26 relative to un-diffracted beam 27 asthe concentration of the analyte in the fluid passing through channels22 changes (i.e. as the fluid passes through the channels).

[0042] If an analyte carried by fluid 22 changes the absorptioncharacteristics of the fluid, but the fluid and analyte together havethe same refractive index of component 12, then only un-diffracted beam27 emerges upon exposure of the system to electromagnetic radiation, butthe intensity of beam 27 changes with the concentration of the analyte.

[0043] The present invention, in one aspect, involves the recognitionthat a change in absorption, and refractive index, of a sample passingthrough channels 22 can be determined simultaneously. This can beaccomplished by providing an electromagnetic radiation detector(photodetector) 28 positioned in line with un-diffracted beam 27, and asimilar detector 30 positioned in line with any of the diffracted beams26, for example in line with beam −1 as illustrated. These can beconnected to a processor 32 constructed and arranged to measure theintensity of electromagnetic radiation striking detector 28, and theintensity of electromagnetic radiation striking detector 30.

[0044] Where the refractive index of fluid passing through channels 22of system 10 changes, this can be determined at processor 32 bydetermining a change in the ratio of intensity of electromagneticradiation striking detectors 28 and 30, respectively. Where theabsorption of fluid passing through channels 22 changes, this can bedetermined at processor 32 by determining a change in totalelectromagnetic radiation striking detectors 28 and 30, combined. Thus,refractive index and absorption of fluid passing through channels 22 canbe determined, simultaneously, as can any change in refractive index orabsorption.

[0045] Thus, a first diffraction pattern can be established with system10 resulting from interaction of electromagnetic radiation 24 with thesystem including separated portions of a first sample residing inadjacent channels 22. The portions are isolated from each other and noelectromagnetic radiation interacts with the sample between theseparated portions. The first sample can be changed to a second sample,i.e., by urging a fluid carrier through channels 22. In this arrangementthe first sample is defined by portions of fluid in channels 22 in linewith incident electromagnetic radiation beam 24, and the second sampledefines a second portion of the sample in channels 22 in line with beam24 after the sample moves within the channels. When the fluid moveswithin the channels, and the first sample is changed to the secondsample, the difference between a first and second diffraction patterncan be determined by determining the difference in ratio ofelectromagnetic radiation striking detectors 28 and 30, respectively.

[0046]FIG. 2A illustrates a top view of system 10 where FIG. 2 is across-section taken through lines 2-2 of FIG. 2A. In FIG. 2A a sampleinlet channel 34 and a sample outlet channel 36 are illustrated whicheach fluidly connect to each of channels 22 and are constructed todeliver fluid to and remove fluid from channels 22. It is emphasizedthat the system as illustrated in FIG. 2 is schematic only. In preferredembodiments, fluid is delivered to each of channels 22 collaterally.That is, if fluid delivered by channel 34 increases in concentration ofa particular analyte at a particular rate, then system 10 should beconstructed, in terms of fluid dynamics, to position fluid in each ofchannels 22 that is identical in concentration of the analyte atidentical locations, axially, within each channel.

[0047] A pump can be provided to deliver fluid sample to and urge fluidsample through the sample chamber. The pump can be a physical pump, suchas a pump that applies pressure to fluid delivered to inlet channel 34,or applies vacuum to fluid exiting via channel 36. A peristaltic-typepump also can be provided where component 12 is flexible or, especially,where component 12 is elastomeric. That is, a series of rollers cancontinuously engage component 12 and compress the component downwardly,decreasing the cross-section of delivery channel 34 or exit channel 36,and the rollers can continuously be urged in the direction of deliveryor removal to provide continuous flow through the channels.

[0048] Alternatively, an electroosmotic pump or pumps can be provided.In one embodiment this is represented by electrodes 38 and 40 providedin inlet channel 34 and exit channel 36, respectively, each addressed byelectrical system circuitry 42 including an electric power source 44.Alternatively, a plurality of electrodes (not shown) can be provided ineach of channels 22 to create electroosmotic pressure. In each caseelectrodes can be fabricated in component 20 via standardmicrofabrication techniques. That is, component 20 can be a chip,covered by component 12 which is positioned to provide channels inregister with electrodes.

[0049] System 10 can be created in any way that provides a samplechamber that can provide a sample in separate, isolated portions suchthat electromagnetic radiation interaction with the system can causediffraction, and such that absorption of the sample can be measured.That is, the system can include a one-piece container including aplurality of separate, elongate, essentially parallel channels, orcomponent system as illustrated. In the system illustrated, components20 and 12 can be made of any materials that are at least somewhattransparent to the electromagnetic radiation. Materials such as glass,quartz and polymers are contemplated. It is important only that a sealbe created between outward-facing surfaces of protrusions 16 andportions of surface 20 in register therewith to allow fluid to becontained within samples 22. Components 12 and 20 can be adhered to eachother via fluid-resistant adhesive, or the like.

[0050] In one set of preferred embodiments component 12 is a polymericarticle, preferably an elastomeric article. One technique forfabrication of article 12 involves casting a polymeric article 12 on acontoured surface created via photolithography including a plurality ofprotrusions complementary to indentations 18 created in article 12.Contact-mode photolithography for this purpose can be carried out and asdescribed in U.S. Pat. No. 5,512,131 of Kurnar, et al., Internationalpatent publication number WO 96/29629 of Whitesides, et al., publishedJun. 2, 1996, U.S. patent application Ser. No. 08/676,951, field Jul. 8,1996 which is co-pending and commonly-owned, and co-pending,commonly-owned U.S. patent application Ser. Nos. 08/853,050 (Rogers, etal., filed May 8, 1997) and 08/873,191 (Qin, et al., filed Jun. 11,1997, each incorporated herein by reference. In one technique asubstrate is coated on one surface with a film of photoresist. Viastandard photolithographic techniques, a pattern is created in the filmof photoresist, the pattern corresponding the desired patterns ofindentations 18. Transparent, elastomeric article 12 then is cast on thesurface defined by the substrate and pattern of photoresist.

[0051] Component 12 can be cast from any precursor that, whensolidified, will create an article that is transparent orsemi-transparent to electromagnetic radiation of interest, a contouredsurface that is conformable and/or includes indentations. “Transparent”or “semi-transparent” means that the article is sufficiently transparentto radiation to render the article useful for sensor or actuatortechniques.

[0052] In one embodiment, component 12 is cast from a polymericprecursor having linear or branched backbones that are cross linked ornon-cross linked, depending upon the particular polymer and degree offormability desired. A variety of elastomeric materials are suitable,especially polymers of the general classes of silicone polymers, epoxypolymers, and acrylate polymers. Epoxy polymers are characterized by thepresence of a three-member cyclic ether group commonly referred to as anepoxy group, 1,2-epoxide, or oxirane. For example, diglycidyl ethers ofbisphenol A may be used, in addition to compounds based on aromaticamine, triazine, and cycloaliphatic backbones. Another example includesthe well-known Novolac polymers. Examples of silicone elastomerssuitable for use as component 12 include those formed from precursorsincluding the chlorosilanes such as methylchlorosilanes,ethylchlorosilanes, phenylchlorosilanes, and the like. A particularlypreferred silicone elastomer is polydimethylsiloxane. Exemplarypolydimethylsiloxane polymers include those sold under the trademarkSylgard by the Dow Chemical Company, Midland, Mich., and particularlySylgard 182, Sylgard 184 and Sylgard 186.

[0053] While casting component 12 from a prepolymeric precursor on asurface patterned with photoresist is preferred, other techniques, suchas casting component 12 from a surface patterned via machining, asurface patterned itself via casting on a photoresist-patterned surface,and the like are suitable. It is important only that component 12 befabricated so as to include a plurality of protrusions 16 andindentations 18 that satisfy criteria that will become apparent.

[0054] In another embodiment component 12 is not flexible or elastomericin its entirety, but the outward-facing surfaces of protrusions 16 arepolymeric and preferably elastomeric and conformable to surface 20. Inthis way, if surface 20 includes imperfections in contour, component 12and component 20 can easily form a seal between them.

[0055] Determining whether a particular material will be suitable foruse in the invention can be readily determined by those of ordinaryskill in the art. Conformability is routinely determined. Selection ofmaterial, in conjunction with selection of an electromagnetic radiationsource, can be carried out by measuring an absorption spectrum of thematerial at the wavelength of the radiation source. If the articlepasses radiation produced by the source at a wavelength that isdetectable (for sensor and display embodiments), then the material andradiation source are suitable for use in the invention.

[0056] The system of the invention defines a reconfigurable diffractiongrating based on a microfluidic device. The array of channels 22 ofFIGS. 1 and 2 and 2A can be fabricated to be approximately 50 micronswide and approximately 20 microns depth, although width and depth can bevaried widely. Samples introduced into channels 22 can be fluids (gas,aqueous solutions, organic solutions, suspensions, and the like). Thedifference in index of refraction between the fluid in the array ofmicrochannels and the structural elastomeric solid generates adifference in the phase of light passing through the device; absorptionby the fluid changes the amplitude of the light. Both of these effectsgive rise to diffraction. Reconfiguration occurs when liquids withdifferent indices of refraction and optical densities are pumped throughthe microchannels. Depths of modulation on the order of around 20 dB andswitching times of around 50 ms are observed. The volume of liquidsampled by incident light in examples below is about 8 nL. The depth andwidth of microchannels 22 can be varied significantly between about 1and 1000 microns.

[0057] When the diffraction pattern is measured, the absorption and thephase are determined. although it is straightforward given theabsorption and the depth of the channels to determine the concentrationor the molar absorption coefficient, to interpret the phase is slightlymore complex because everything is mod 2 pi (i.e. phase gratings thatshift the phase by 2 pi look the same as those that shift the phase by 4pi.) In many cases, monitoring changes in the phase will be sufficientbecause the changes are often very small compared to 2 pi. in othercases, however, it may be important to determine the index of refractionand not just the phase. There are several ways to determine the index ofrefraction from the diffraction pattern: (i) measure the diffraction atseveral laser wavelengths, or (ii) at a single laser wavelength, measureseveral gratings that have different channel depths, (iii) at a twowavelengths that are not integral multiples of one another, measure thediffraction pattern as a function of number of passes through thegrating or (iv) some combination of (i), (ii) and (iii). In oneembodiment a grating is constructed with channel depths that vary in acontinuous way along the channels. Probing this type of structure atseveral (or many) points gives the dependence of the diffraction patternon the channel depth, and yields the information required to determinethe index accurately. One can achieve the necessary structure by thestandard molding article 12 against an appropriate master. Or, one canform the structure by applying spatially varying force to a flexiblearticle 12 that in its undeformed state has channel depths that do notvary along their length.

[0058] Devices described herein can be used as sensing elements inmicrototal analysis systems and as actuators. The systems of theinvention can be used in sensing, that is, real-time monitoring of thevariation of index of refraction and absorption of a fluid (seeHarrison, et al., Sensors and Actuators B-Chemical, 33 (1996) 105-109,incorporated herein by reference; O. Solgaard, et al., “Deformablegrating optical modulator”, Opt. Lett., 17 (1992) 688-690; A. van denBerg, et al., “Micro Total Analysis Systems, Kluwer, Dordrecht, 1995; A.Manz, et al., “Microsystem technology in chemistry and life science”,Springer, Berlin, 1998; E. M. J. Verpoorte, et al., “Three-dimensionalmicro flow manifolds for miniaturized chemical analysis systems”, J.Micromech. Microeng., 4 (1994) 246-256.

[0059] A sensor based on this device can be used to monitor thevariations in the optical properties (index of refraction andabsorption) of a fluid by measuring the intensities of diffracted beams.The switching time of this type of device (determined by the speed atwhich fluids can be pumped through the channels) is in the range of fromabout 1 to about 500 ms and allows it, for example, to be used inreal-time monitoring of the evolution of a chemical reaction on a chip.Furthermore, the liquids in the channels can be pumped by electroosmosisas described above. This possibility makes the device compatible withmany μTAS that use electric fields to pump liquids and to separateanalytes. The detection of fluids by diffraction is sufficientlysensitive according to the invention to offer a simple and cheapalternative to detection that is inherently compatible with μTAS.

[0060] Also, these devices can be used in actuating, for examplewavefront engineering, reconfigurable optical filters, and modulators(see, L. Buydens, et al., “Amplitude modulation and bea-steeringproperties of active binary phase gratings with reconfigurableabsorption areas”, Appl. Opt. 33 (1994) 4792-4800). Fluids withdifferent optical properties can be pumped through the channels tomodulate the diffraction of light. Complex modulations of diffractioncan be achieved if the channels are addressed independently and filledwith fluids with different optical properties. Microfluidic devicesbased on this concept can find applications in spatial light modulationor wavefront engineering, and can be used in beam-steering devices orphase arrays. This type of device is complementary toelectrostatically-modulated diffraction gratings.

[0061] One aspect of the invention involves a technique for treating asurface to promote sealing of the surface to another surface or chemicalmodification of the surface in way that can promote fluid flow within afluid channel defined in part by the surface. In a technique forpromoting sealing, the invention involves sealing a first surface to asecond surface where the first surface, the second surface, or both canbe well-sealed to form a fluid-tight seal (a seal impermeable to fluidsthat are not destructive, i.e. do not dissolve or otherwise degrade,components joined to form the seal) in the absence of auxiliaryadhesive. Auxiliary adhesive, in this context, means an agent, separatefrom components joined to form a seal, coated on one or both surfaces ofthe components at locations joined to form the seal. The techniqueinvolves, in one set of embodiments, pre-oxidizing the first, thesecond, or both surfaces prior to joining them. This is particularlyuseful when at least one surface is a polymeric surface. For example, apolymeric surface can be pre-oxidized and adjoined to a second surface,such as another polymer, glass or the like, that also is pre-oxidized,and the first and second surfaces can be joined and allowed to form aliquid-impermeable seal therebetween. Pre-oxidation can be carried outby exposing the surfaces to plasma and, where one or both surfacesexpose silicon atoms, pre-oxidation of the surface can result information of siloxane bonds between the two surfaces when they arejoined. For example, a silicon-containing polymer such as PDMS, whenpre-oxidized via exposure to plasma, defines a conformable surface thatcan form a liquid-impermeable seal with another surface such as glass orPDMS via formation of siloxane bonds therebetween. It is particularlyuseful to select at least one component that is conformable (for examplean elastomeric polymer) to form such a seal because in the absence ofauxiliary adhesive, good surface contact between the two componentsgreatly enhances good seal formation.

[0062] It is a feature of sealing techniques of the invention, involvingpre-oxidation of polymeric or other surfaces via, for example, exposureto plasma and formation of siloxane bonds, that liquid-impermeable sealscan be formed at or near room temperature. Specifically, a polymericsurface can be sealed to a second surface to form a liquid-impermeableseal, in the absence of auxiliary adhesive, at a temperature of betweenabout 12° C. and about 34° C., more preferably from about 16° C. toabout 27° C., more preferably still from about 18° C. to about 23° C.For a discussion of chemistry of plasma-induced sealing via formation ofsiloxane bonds, reference is made to Madou, “Fundamentals ofMicrofabrication,” CRC Press, New York, pgs. 382-395, incorporatedherein by reference.

[0063] Particularly relevant to fluid flow within channels, as describedabove, techniques of the invention advantageously can involve sealingoutward-facing surfaces of protrusions 16 to surface 20 that does notrequire auxiliary adhesive. This technique can be applied to any of avariety of systems, not necessarily those systems described herein. Thistechnique involves exposing outward-facing surfaces of protrusions 16(optionally all of surface 14) to plasma, and then contacting firstportions of the first surface (outward-facing surfaces of theprotrusions) with the second surface while leaving a second portion ofthe first surface (indentation 18), intervening the first portions ofthe first surface, free of contact with the second surface. A seal iscreated in this manner that is impermeable to fluids that are notdestructive (do not dissolve or otherwise degrade) components 12 or 20,in the absence of auxiliary adhesive.

[0064] Plasma treatment to pre-oxidize a polymeric surface can be usedaccording to another aspect of the invention involving inducingelectroosmotic fluid flow in a channel where an interior surface of thechannel is defined at least in part by the polymeric material. Withreference to FIGS. 1-3, specifically FIG. 2 for purposes of thisdiscussion, this can involve exposing surface 20 of component 14 toplasma, and exposing surface 14 of component 12 to plasma, wherecomponents 12 and 14 are made of material that will expose a chargedsurface upon exposure to plasma. For example, where surfaces 20 and 14expose silicon atoms, e.g. where article 20 and 12 are glass or PDMS,exposure to plasma will result in exposed oxygen atoms that quicklybecome hydroxyl groups that carry a negative charge that can facilitateelectroosmotic flow. Electroosmotic flow can be facilitated usingelectrical circuitry positioned to apply an electrical field along anyone or all of channels 22 formed when articles 12 and 20 are joined toeach other. Such electrical circuitry can readily be arranged by thoseof ordinary skill in the art and is not shown.

[0065] Another aspect of the invention involves chemical modification atsurfaces promoted by a pretreatment step that can be the same step usedto pre-oxidize a surface for bonding to another surface, or modificationof a surface to facilitate electroosmotic flow. With reference to FIG.2, exposure of surfaces 14 and 20 to plasma not only pre-oxidizes thesurfaces such that bonding of surface 14 to surface 20 is promoted inthe absence of auxiliary adhesive, but primes the interior surfaces ofthus-formed channels 20 to chemical modification. Once channels 22 aredefined by joining outward-facing surfaces of protrusions 16 to portionsof surface 20 in register therewith, the interior surfaces of channels22 can be modified to expose a predetermined chemical functionality. Inone embodiment, the interior surfaces of channels 22 are modified bycoating them with a chemical that bonds to the interior surface afterthe pretreatment step and that facilitates fluid flow within thechannel. That is, fluid flow can be urged within the channels at a firstfluid flow rate under conditions at which, had the pretreatment step notbeen carried out, the fluid would flow at a slower rate.

[0066]FIG. 11 illustrates schematically system 10 of the invention.Although not illustrated, each of channels 22 can be addressed by adifferent fluid source. This is routine to those of ordinary skill inthe art. When fluids of different refractive index are passed throughthe different channels 22, the effect on passage of light, from source24, can be essentially identical to a situation in which the depth ofthe channels varies, as illustrated in FIG. 12.

[0067]FIG. 13 illustrates system 10 through lines 13-13 of FIG. 11, inwhich channels 22 are not only filled with different fluids, but thecontent of the fluids changes axially along the channels. That is,sections of different fluid samples can be injected into each channel asdesired. This creates “blocks” 50, 52, 56, 58, 60, 62, 64, 66, etc.,that can be positioned as desired in any channel and at any positionaxially in the desired channel. This technique is best carried out usingplugged-flow electroosmotic pressure to urge flow in the channels asdescribed above. For example, a carrier channel can be provided withbranches that supply samples. Electrodes can be provided to urge samplesinto the carrier channel in appropriate sequence, and a control systemcan be provided to activate the electrodes when required. The system ofFIG. 13, when exposed to electromagnetic radiation, produces atwo-dimensionally variant pattern. That is, different samples havingdifferent refractive indices and/or absorption characteristics can bepositioned as desired to generate a display (diffraction pattern, etc.),to serve as a detector, etc.

EXAMPLES

[0068] Elastomeric diffraction gratings were prepared by casting PDMSprepolymer (Sylgard 184, Dow Corning) onto a patterned photoresistsurface relief (the master) generated by photolithography. A. Kumar andG. M. Whitesides, Features of gold having micrometer to centimeterdimensions can be formed through a combination of stamping with anelastomeric stamp and an alkanethiol ink followed by chemical etching,Appl. Phys. Lett., 64 (1993) 2002-2004; A. Kumar, H. A. Biebuyck, and G.M. Whitesides, Patterning self-assembled monolayers: applications inmaterial science, Langmuir, 10 (1994)1498-1511. The pattern ofphotoresist consisted of an array of 20 parallel lines, 20 μm thick, 50μm wide, and separated by 50 μm. The lines of photoresist were 2.5 cmlong, and connected to a 4×4 mm² pad at each end: the pads definedoutlines for reservoirs to which fluids could be added. After curing for1 h at 65° C., the polymer was removed from the master to give afree-standing PDMS mold with an array of microchannels embossed on itssurface: this array serves as a diffraction grating. Reservoirs forliquids were cut out through the thickness of the PDMS slab.

[0069] The choice of the critical dimensions (depth and width) of themicrochannels requires comment. The depth of the channels is determinedby the thickness of the photoresist on the master: this thickness can becontrolled by adjusting the speed of spin-coating of the photoresist inthe photolithographic step. We used Shipley 1110 photoresist coated at1500 rpm to produce films 20 μm thick; using different spin speeds andphotoresists, channels ranging in thickness from 1 μm to 1000 μm couldbe produced easily. (To make devices with channels thicker than 20 μm,different photoresists have to be employed, e.g., SU-8 50 and SU-8 250(Microlithography Chemical Corp.).) The width of the microchannels isdetermined by the mask used in photolithography: we used a highresolution transparency as a mask that places a lower bound of 20 μm onthe width of the lines. D. Qin, Y. Xia, and G. M. Whitesides, Rapidprototyping of complex structures with feature sizes larger than 20 μm,Adv. Mat., 8 (1996) 917-919. By using commercial chrome masks, however,microchannels with widths and separations as small as 1 μm can befabricated. We chose to fabricate devices with relatively wide channels(50 μm) because the production of transparencies is much cheaper andfaster than that of chrome masks, and because it enables faster flow offluid that reduces the switching time of the device.

[0070] To form enclosed microchannels, the microfluidic devices weresealed in the following way. First, the PDMS mold and a glass slide wereplaced in a plasma oxidation chamber and oxidized for 1 minute. The PDMSstructure was then placed on the glass slide with the surface relief incontact with the glass. The conformal contact between the mold and glassresulted in an irreversible seal between the two substrates: the bondbetween the two substrates was so strong that the PDMS mold could not bepeeled from the glass substrate without damaging the elastomer. Theirreversible seal is a result of the formation of bridging siloxanebonds (Si—O—Si) between the two substrates that result from acondensation reaction between silanol (SiOH) groups that are present atboth surfaces after plasma oxidation. FIG. 1 shows a picture of a PDMSmold, with an array of microchannels defined on its surface (inset),sealed against a glass slide.

[0071] Diffraction of light by the microchannels was controlled byfilling them with different solutions of either inorganic salts or dye.Solutions of sodium chloride (NaCl, EM Science), potassium thiocyanate(KSCN, Aldrich), and Azure A (Aldrich) were prepared in deionized waterand thermally equilibrated at room temperature prior to use; solutionsof dye were filtered to remove particulates. The ranges ofconcentrations were as follows: NaCl: 0-20% (w/w); KSCN: 30-55% (w/w);Azure A: 0-100 mM. The index of refraction of the solutions of inorganicsalts (n_(f)) was estimated using tabulated data (R. C. Weast, Ed. CRCHandbook of Chemistry and Physics, 66th ed., 1985: the indices ofrefraction in these tables were measured at 589 nm and 20° C. Ourdiffraction experiments were performed at 633 m and at room temperature(˜22° C.): the index of refraction in our experiments and the tabulateddata may differ by 0.002-0.004. We are concerned, however, withdifferences in phase rather than absolute values of phase so smallerrors in n_(f) do not effect greatly the interpretation of our data.):n_(f) ranges from 1.333 to 1.386 for the NaCl solutions, and from 1.393to 1.452 for the KSCN solutions. The microchannels were filled byplacing a drop of fluid in one reservoir and pulling vacuum from theother end of the microchannels. The microchannels were emptied byapplying vacuum and were thoroughly rinsed with deionized water betweenmeasurements.

[0072] Characterization of the devices as diffraction gratings wasperformed with a He—Ne laser (633 nm). The incoming laser beam wascentered on the array of microchannels, and the intensity of thediffracted beams was recorded using a photodiode (Newport, Model 818-SL)and a power meter (Newport, Model 1830-C) positioned ˜2 m from thegrating: this distance allowed spatial resolution of the diffractedbeams. The diameter of the laser beam at the grating was ˜1 mm:approximately 10 microchannels (FIG. 1) and a volume of liquid of ˜8 nLwere sampled in the diffraction experiments. Specifically, FIG. 1 showsa microfluidic diffraction grating. A PDMS mold containing an array of20 parallel microchannels (50 μm wide, 20 μm deep, spaced by 50 μm) wassealed against a glass substrate. A dye was pumped through the channelsfrom the reservoirs to provide contrast. The inset shows a magnifiedimage of the channels; the width of this array is ˜2 mm. In thediffraction experiments, the diameter of the laser beam at the gratingwas ˜1 mm: approximately 10 microchannels and a volume of liquid of 8 nLwere therefore sampled.

[0073] We investigated the influence of the index of refraction of asolution on the diffraction pattern generated by the microfluidicdevice. The intensities of 0th and 1st order beams of the diffractionpattern from an array of microfluidic channels filled with solutions ofNaCl are plotted in FIG. 8 as a function of the concentration of salt(The diffraction pattern of the empty PDMS grating remained constantthroughout the experiments, indicating that the PDMS was not swelled byaqueous solutions.). The insertion loss of ˜0.1 dB shows that absorptionand reflection by PDMS and glass have a small effect on the performanceof the device. These data show a modulation of the diffraction when theconcentration of NaCl increases. The changes in intensity of the 0th and1st diffracted orders are large, i.e., ˜20 db; this depth of modulationcompares favorably to that (16 dB) of an electrostatically actuateddiffraction grating that was micromachined in silicon. The solid linesin FIG. 8 represent a fit to these data using the plots generated inFIGS. 6 and 7 and assuming zero absorption. The good agreement betweentheory and experiment indicates that the modulation of the diffractionpattern is consistent with changes in the phase as the index ofrefraction of the liquid in the microchannels was varied, i.e., thedevice operated as a phase grating. The overall modulation was 1.1-1.2cycles (i.e., 2.3π in phase angle) for NaCl solutions varying from 0-20%(w/w).

[0074] A similar experiment was carried out using aqueous solutions ofKSCN. In this case, index-matching of PDMS was achieved when themicrochannels were filled with a solution containing ˜38% (w/w) KSCN:this solution has an index of refraction of ˜1.41 (Weast, above).

[0075] The influence of absorption on the diffraction pattern generatedby the microfluidic device was investigated by filling the microchannelswith aqueous solutions of a dye, Azure A, that has a maximum inabsorption (ε˜10⁵ M⁻¹cm⁻¹) at 633 μm. The variations in the intensitiesof the 0th and 1st order beams of the diffraction pattern as a functionof the concentration of dye are shown in FIGS. 9 and 10. FIG. 9 showsthat the modulation of the intensity of the diffracted beams was onlypartial: neither the 0th nor the 1st order diffracted beams becametotally illuminated or extinct as the concentration of dye was changed.This behavior is consistent with differences between both the amplitudeand phase of the light passing through PDMS and the fluid in themicrochannels FIGS. 6 and 7.

[0076] Both the absorption and the phase of the light passing throughthe diffraction grating can be determined simultaneously. FIG. 10 showsthe variation of the transmission of the device, determined by summingthe intensity of the first six diffracted orders (0 to ±5), as afunction of concentration. As expected, the transmission decreased asthe concentration of dye in solution was increased but, since themicrochannels that contain the absorbing fluid represent only half ofthe surface area of the grating, the lower value of transmission of thedevice was only 0.5. The phase can be determined from the relativeintensity of the 0th and 1st order diffracted beams, the measuredtransmittance of each solution, and the calculated plots shown in FIGS.6 and 7: the phase determined for each solution is shown in FIG. 10. Thephase increases linearly with the concentration of dye in solution.

[0077] The results presented in this section indicate that this opticalmicrofluidic device could be used to monitor changes in the index ofrefraction and absorption of a fluid. This device could potentially beuseful in applications that require dynamic, real-time monitoring of theevolution of the properties of a liquid, for example, during chemicalreactions in μTAS. In the device presented here, for a laser beamdiameter of ˜1 mm, a volume of liquid of 8 nL was sampled: this volumeapproaches those encountered in μTAS. This grating is amenable toseveral methods of pumping the liquids through the microchannels thatwould provide time resolution during a dynamic measurement of thediffraction pattern: application of vacuum and pressure, andelectroosmotic pumping.

[0078] The switching time of the device is equal to the time it takesfor fluid in the volume illuminated by the laser beam to be displaced.Application of vacuum caused the fluid in the 2.5 cm long microchannelsto be completely displaced in less than 1 s: this pumping rate thereforeresults in a switching time of about 50 ms in the region sampled by thelaser beam. If the liquids were pumped by pressure or electroosmosisthen the switching time would be longer for the experimental conditionscommon in μTAS: for example, electroosmotic pumping in this devicecaused by applying an electric field of 500 V/cm across the 20microchannels would result in a switching time of ˜500 ms. FIG. 8 showsvariation in the intensities of the 0th (filled circles) and 1st (opendiamonds) order beams of the diffraction pattern from a microfluidicgrating as a function of the concentration of NaCl in solutions fillingthe microchannels. The measurements were carried out at 633 nm. The datafor the 1st order beams are an average of the positive and negativebeams; both sets of data were normalized to the total intensity of lighttransmitted through the device. A second x-axis is labeled that showsthe index of refraction of the solutions determined from tabulatedvalues (Weast, R. C., above). The index of refraction varies linearlywith concentration from 1.333 for pure water to 1.3684 for a 20% NaClsolution: this difference in index of refraction corresponds to ˜1.1cycle of modulation (Eq. 2). The solid lines show the variation of theintensities of the 0th and 1st orders for a pure phase grating as afunction of the phase difference (top x-axis) calculated from Eq. 1. Thechange in phase over the range of concentrations determined from the fitis ˜2.37π, i.e., ˜1.2 cycles of modulation. The modulation of themicrofluidic diffraction grating is therefore consistent with a changein phase induced by a variation solely in the index of refraction of thefluid filling the microchannels. The deviation in the experimental datafrom the fits probably arises from instabilities in the He—Ne laser anddetector that result in errors of ±5% in the measurements.

[0079]FIG. 9 shows variation in the intensities of the 0th (closedcircles) and 1st (open circles) orders of the diffraction pattern from amicrofluidic grating as a function of the concentration of Azure A inaqueous solutions. FIG. 10 shows variation of transmission of themicrofluidic device (open circles) and phase (closed circles) as afunction of concentration of Azure A. The phase was determined from fitsto Eq. 1 of the diffraction pattern for each solution of dye; the solidline through these data represent a linear fit. The dotted line is aguide to the eye for the transmission of the device. The transmittanceof the liquid in the channels was fitted to the Beer-Lambert law (notshown). The low value of Ed (˜11 M) produced by this fit compared to thevalue estimated from the coefficient of extinction of Azure A and thethickness of the microchannels (εd=200 M) can be explained by thedeviation from the ideal behavior described by the Beer-Lambert law athigh concentrations (>10 mM) of an absorbing species.

[0080] Those skilled in the art would readily appreciate that allparameters listed herein are meant to be exemplary and that actualparameters will depend upon the specific application for which themethods and apparatus of the present invention are used. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that, within the scope of the appended claimsand equivalents thereto, the invention may be practiced otherwise thanas specifically described.

What is claimed is:
 1. A method comprising: establishing a firstelectromagnetic radiation diffraction pattern resulting from interactionof electromagnetic radiation with a system including separate portionsof a first sample; changing the first sample to a second sample andestablishing a second diffraction pattern resulting from interaction ofelectromagnetic radiation with the system including a second sample; anddetermining the difference between the first and second diffractionpatterns.
 2. A method comprising: exposing separate portions of a firstsample to electromagnetic radiation, without exposing any portion of thefirst sample between the separate portions to the electromagneticradiation, and determining absorption of the electromagnetic radiationby the first sample; exposing separate portions of a second sample toelectromagnetic radiation, without exposing portions of the secondsample between the separate portions to the electromagnetic radiationand determining absorption of the electromagnetic radiation by thesecond sample; and determining a difference in absorption of the secondsample as compared to the first sample.
 3. A method as in claim 2,further comprising establishing a first diffraction pattern resultingfrom interaction of electromagnetic radiation with a system includingthe separate portions of the first sample; establishing a seconddiffraction pattern resulting from interaction of the electromagneticradiation with the system including the second sample; and determiningthe difference between the first and second diffraction patterns.
 4. Amethod as in any preceding claim, wherein at least one of the first andsecond samples is two-dimensionally variant.
 5. A method as in anypreceding claim, wherein the separate portions of the first sample areisolated from each other, and the separate portions of the second sampleare isolated from each other.
 6. A method as in any preceding claim,wherein the separate portions of the first sample are exposed toelectromagnetic radiation without exposing any portion of the firstsample between the separate portions to the electromagnetic radiation,and separate portions of the second sample are exposed toelectromagnetic radiation without exposing portions of the second samplebetween the separate portions to the electromagnetic radiation.
 7. Amethod as in any preceding claim, involving determining absorption ofthe electromagnetic radiation by each of the first and second samplesand determining a difference in absorption of the second sample comparedwith the first sample.
 8. A method as in claim 7, involvingsimultaneously determining the first diffraction pattern and absorptionof the first sample, and simultaneously determining the seconddiffraction pattern and absorption of the second sample.
 9. A method asin any preceding claim, wherein the first and second samples aredifferent fluids.
 10. A method as in claim 9, wherein the first andsecond samples contain different concentrations of a species.
 11. Amethod as in claim 9, wherein the first and second samples containdifferent species.
 12. A method as in claim 9, wherein the first andsecond samples differ in absorption.
 13. A method as in claim 12,wherein the first and second samples differ in refractive index.
 14. Amethod as in claim 9, wherein the first and second samples differ inrefractive index.
 15. A method as in any preceding claim, wherein eachof the first and second samples comprises a series of elongate,essentially parallel sections.
 16. A method as in claim 15, wherein eachof the first and the second samples is a different fluid.
 17. A methodas in claim 15, wherein the elongate sections comprise a series ofdifferent blocks of fluid.
 18. A method as in any preceding claim,wherein each of the first and second samples is two-dimensionallyvariant, and the diffraction pattern is two-dimensionally variant.
 19. Amethod as in claim 15, further comprising urging the sections in anaxial direction thereby positioning the first sample at an axiallocation, and subsequently positioning the second sample at the sameaxial location.
 20. A method as in claim 19, involving urging thesections in an axial direction via physical pressure.
 21. A method as inclaim 19, comprising urging the sections in an axial direction viaelectroosmosis.
 22. A method as in any preceding claim, wherein each ofthe first and second samples is positioned in elongate voids in anarticle that is at least partially transparent to the electromagneticradiation.
 23. A method as in claim 22, wherein each of the first andsecond samples is positioned in indentations in a first chambercomponent including a plurality of protrusions and interveningindentations, the protrusions being sealed to a surface of a secondchamber component.
 24. A method as in any preceding claim, wherein eachof the first and second samples is positioned in isolated, essentiallyparallel channels in a sample chamber.
 25. A method as in claim 23,wherein the sample chamber includes at least one interior sample surfacethat is flexible.
 26. A method as in claim 23, wherein the samplechamber includes at least one interior sample surface that is polymeric.27. A method as in claim 23, wherein the sample chamber includes atleast one interior sample surface that is elastomeric.
 28. A method asin any preceding claim, wherein each of the first and second samples ispositioned in isolated, essentially parallel channels in a samplechamber that is essentially transparent to the electromagneticradiation.
 29. A method comprising: exposing a first surface of a firstcomponent and a second surface of a second component to plasma; andcontacting first portions of the first surface with the second surfacewhile leaving a second portion of the first surface, intervening thefirst portions of the first surface, free of contact with the secondsurface.
 30. A method as in claim 29, the contacting step comprisingcontacting a plurality of first portions of the first portions of thefirst surface, each of the first portions including a second portionthat remains free of contact with the second surface, between it andanother first portion.
 31. A method as in claim 30, wherein the firstportions are polymeric.
 32. A method as in claim 30, wherein the firstportions are elastomeric.
 33. A method as in claim 29, wherein the firstsurface is a contoured surface including a plurality of protrusions andintervening indentations and the contacting step involves contactingoutward-facing surfaces of the protrusions with the second surface. 34.A method as in claim 33, wherein the second surface is essentially flat.35. A method as in claim 29, further comprising forming a seal betweenthe first portions and the second surface.
 36. A method as in claim 35,involving forming an irreversible seal between the first portions andthe second surface.
 37. A method as in claim 35, involving forming aseal between the first portions and the second surface that isimpermeable to agents to which the first and second surfaces areresistant.
 38. A method as in claim 29, wherein at least one of thefirst and second components is flexible.
 39. A method as in claim 28,wherein at least one of the first and second components is polymeric.40. A method as in claim 29, wherein at least one of the first andsecond components is elastomeric.
 41. A method as in claim 29, thecontacting step comprising defining, between the first surface and thesecond surface, a plurality of isolated, essentially parallel, elongatechannels.
 42. A method as in claim 41, wherein each of the channels hasa length at least three times its width.
 43. A method as in claim 41,wherein the plurality of channels comprises at least five channels. 44.A method comprising: providing a sample chamber formed according toclaim 29; and positioning a source of electromagnetic radiation directedat the sample chamber and a detector of electromagnetic radiationpositioned to detect electromagnetic radiation emanating from the samplechamber.
 45. A method as in claim 44, comprising positioning thedetector to detect electromagnetic radiation emitted by the emitter andpassing through the sample chamber.
 46. A method as in claim 45, whereinthe detector is an electromagnetic absorption detector.
 47. A method asin claim 45, wherein the detector is a diffraction pattern detector. 48.A method as in claim 44, further comprising providing a diffractionpattern detector positioned to detect a diffraction pattern resultingfrom interaction of the electromagnetic radiation and a sample in thesample chamber.
 49. A method as in claim 44, further comprisingproviding a pump constructed and arranged to urge a sample through thechamber.
 50. A method as in claim 49, wherein the pump is a physicalpump.
 51. A method as in claim 49, wherein the pump is an electroosmoticpump.
 52. A system comprising: a sample system constructed and arrangedto position first and second portions of a sample separately and inisolation from each other; at least one source of electromagneticradiation positioned to irradiate the first and second portions; and atleast one absorption detector positioned to detect absorption of thefirst and second portions.
 53. A system comprising: a sample systemconstructed and arranged to position first and second portions of asample separately and in isolation from each other; a source ofelectromagnetic radiation positioned to irradiate the first and secondportions; a detector positioned to determine diffraction of theelectromagnetic radiation by the first and second portions; and a pumpconstructed and arranged to displace the first sample with a secondsample.
 54. A system as in claim 53, wherein the detector is constructedand arranged to detect a one-dimensional diffraction pattern.
 55. Asystem as in claim 53, wherein the detector is constructed and arrangedto detect a two-dimensional diffraction pattern.
 56. A system as inclaim 52, wherein the sample system comprises a sample chamberconstructed and arranged to position the first and second portions ofthe sample separately and in isolation.
 57. A system as in claim 56,wherein the sample chamber includes an interior chamber surface that isflexible.
 58. A system as in claim 56, wherein the sample chamberincludes an interior chamber surface that is polymeric.
 59. A system asin claim 56, wherein the sample chamber includes an interior chambersurface that is elastomeric.
 60. A system as in claim 53, wherein thesample chamber is formed of a first chamber component and a secondchamber component sealed to each other via plasma activation in theabsence of auxiliary adhesive.
 61. A system as in claim 52, wherein thesample system comprises a sample chamber including a plurality ofessentially parallel, elongate channels.
 62. A system as in claim 61,wherein the channels comprise at least five channels.
 63. A system as inclaim 61, further comprising a pump constructed and arranged to urgesamples through the channels.
 64. A system as in claim 63, wherein thepump is an electroosmotic pump.
 65. A system as in claim 64, wherein theelectroosmotic pump comprises electrodes, in each channel, spacedaxially in the channel.
 66. A system as in claim 65, wherein theelectrodes are pre-fabricated on a chip and the sample chamber isdefined by the chip and a cover on the chip.
 67. A system comprising: asample chamber defined by an elastomeric article having a first surfaceincluding a plurality of protrusions and indentations, outward-facingsurfaces of the protrusions forming a seal against a surface of a secondarticle, the indentations and portions of the surface of the secondarticle defining a plurality of elongate, essentially parallel fluidchannels constructed and arranged to receive a fluid and to pass thefluid through the channels; a pump constructed and arranged to urge asample through the channels; at least one source of electromagneticradiation positioned to irradiate a sample in the sample chamberchannels; a detector positioned to determine absorption ofelectromagnetic radiation directed at a sample in the sample chamberchannels; and a diffraction detector positioned to detect diffraction ofelectromagnetic radiation directed at a sample in the sample chamberchannels.
 68. A system as in claim 67, constructed and arranged tosimultaneously determine absorption of electromagnetic radiationdirected at a sample in the sample chamber channels and diffraction ofelectromagnetic radiation directed at a sample in the sample chamberchannels.
 69. A system as in claim 68, constructed and arranged tosimultaneously determine absorption of electromagnetic radiationdirected at a sample in the sample chamber channels and diffraction ofelectromagnetic radiation directed at a sample in the sample chamberchannels, and to simultaneously determine absorption of electromagneticradiation directed at a second sample in the sample chamber channels anddiffraction of electromagnetic radiation directed at a second sample inthe sample chamber channels and to determine a difference in absorptionof the first sample compared with the second sample and to determine adifference in diffraction of the first sample compared to the secondsample.
 70. A method comprising: joining a pre-oxidized polymericsurface to a second pre-oxidized surface; and allowing the polymericsurface and the second surface to form a liquid-impermeable sealtherebetween.
 71. A method as in claim 70, comprising allowing thepolymeric surface and the second surface to form a liquid-impermeableseal therebetween in the absence of auxiliary adhesive.
 72. A method asin claim 70, further comprising pre-oxidizing the polymeric surface andthe second surface by exposing the polymeric surface and the secondsurface to plasma.
 73. A method as in claim 70, wherein the polymericsurface is a surface of a flexible article.
 74. A method as in claim 70,wherein the polymeric surface is a surface of an elastomeric article.75. A method as in claim 70, wherein the joining step comprises joiningthe first portions of the polymeric surface to the second surface whileleaving a second portion of the polymeric surface, intervening the firstportions of the polymeric surface, free of contact with the secondsurface.
 76. A method as in claim 70, the joining step comprisingcontacting first portions of the second surface with the polymericsurface while leaving a second portion of the second surface,intervening the first portions of the second surface, free of contactwith the polymeric surface.
 77. A method as in claim 70, wherein thesecond surface is polymeric.
 78. A method as in claim 70, wherein thesecond surface is metal.
 80. A method comprising: forming a siloxanebond between a first, conformable surface and a second surface.
 81. Amethod as in claim 80, comprising forming of the siloxane bond in theabsence of auxiliary adhesive.
 82. A method as in claim 80, furthercomprising exposing the first surface and the second surface to plasma,then forming the siloxane bond.
 83. A method as in claim 80, wherein thefirst surface is polymeric.
 84. A method as in claim 80, wherein thefirst surface is flexible.
 85. A method as in claim 80, wherein thefirst surface is elastomeric.
 86. A method as in claim 80, wherein thesecond surface is polymeric.
 87. A method as in claim 80, the formingstep involving contacting first portions on the first surface with thesecond surface while leaving a second portion of the first surface,intervening the first portions of the first surface, free of contactwith the second surface.
 88. A method as in claim 80, the forming stepinvolving contacting first portions on the second surface with the firstsurface while leaving a second portion of the second surface,intervening the first portions of the second surface, free of contactwith the first surface.
 89. A method as in claim 80, wherein the secondsurface is metal.
 90. A method comprising: applying a polymeric surfaceto a second surface; and in the absence of auxiliary adhesive and at atemperature of between about 16° C. and about 27° C., allowing thepolymeric surface and the second surface to bond to form aliquid-impermeable seal therebetween.
 91. A method as in claim 90,further comprising pre-oxidizing the polymeric surface and the secondsurface prior to the applying step.
 92. A method as in claim 90, furthercomprising exposing the polymeric surface and the second surface toplasma prior to the applying step.
 93. A method as in claim 90, whereinthe second surface is polymeric.
 94. A method as in claim 90, whereinthe first surface is flexible.
 95. A method as in claim 90, wherein thefirst surface is elastomeric.
 96. A method as in claim 90, the applyingstep comprising contacting first portions on the first surface with thesecond surface while leaving a second portion of the first surface,intervening the first portions of the first surface, free of contactwith the second surface.
 97. A method as in claim 90, the applying stepcomprising contacting first portions on the second surface with thefirst surface while leaving a second portion of the second surface,intervening the first portions of the second surface, free of contactwith the first surface.
 98. A method as in claim 90, wherein the secondsurface is metal.
 99. An article comprising: a polymeric componenthaving a surface bonded to a surface of a second component in theabsence of auxiliary adhesive thereby defining a liquid-impermeable sealtherebetween.
 100. An article as in claim 99, wherein the surface of thepolymeric component is bonded to the surface of the second component viasiloxane bonding.
 101. An article as in claim 99, wherein first portionsof the surface of the polymeric component are bonded to the surface ofthe second component while a second portion of the surface of thepolymeric component, intervening the first portions of the surface ofthe polymeric component, is free of contact with the surface of thesecond component.
 102. An article as in claim 99, wherein first portionsof the surface of the second component are bonded to the surface of thepolymeric component while a second portion of the surface of the secondcomponent, intervening the first portions of the surface of the secondcomponent, is free of contact with the surface of the polymericcomponent.
 103. An article as in claim 99, wherein the surface of thesecond component is metal.
 104. A method comprising: inducingelectroosmotic fluid flow in a channel, an interior surface of which isdefined at least in part by polymeric material.
 105. A systemcomprising: a channel, an interior surface of which is defined at leastin part by polymeric material; and electrical circuitry positioned toapply an electrical field along the channel.
 106. A method comprising:exposing a surface of a first article to a pretreatment step, absentauxiliary adhesive, that both promotes bonding of the surface to anothersurface and primes the first surface for a predetermined chemicalmodification; defining a channel between the first article and a secondarticle by joining portions of the surface of the first article toportions of a surface of the second article and allowing joined portionsof the first and second articles to bond to form a liquid-impermeableseal therebetween promoted by the pretreatment step; and effecting thepredetermined chemical modification at an interior surface of thechannel primed by the pretreatment step.
 107. A method comprising:exposing a surface of a first article to a pretreatment step, absentauxiliary adhesive, that both promotes bonding of the surface to anothersurface and primes the first surface for enhanced fluid flow against it;defining a channel between the first article and a second article byjoining portions of the surface of the first article to portions of asurface of the second article and allowing joined portions of the firstand second articles to bond to form a liquid-impermeable sealtherebetween promoted by the pretreatment step; and urging fluid flow inthe channel at a first fluid flow rate under conditions at which, in theabsence of the pretreatment step, the fluid would flow at a second fluidflow rate less than the first rate.
 108. A method as in claim 33,comprising forming a plasma-activated seal in the absence of auxiliaryadhesive.